(a) Field of the Invention
The present invention relates to a voltage-controlled oscillator, and more particularly to a voltage-controlled oscillator using a variable inductor.
(b) Description of the Prior Art
A voltage-controlled oscillator (VCO) is an electronic oscillation circuit whose oscillation frequency is controlled by the voltage input. Its oscillation frequency varies with different input DC voltages, and it is often applied to such as frequency modulation (FM) circuits, phase modulation (PM) circuits, or pulse width modulation (PWM) circuits, or the like. An LC resonant circuit comprised of a variable capacitance diode C and an inductor L is usually employed to adjust the oscillation frequency of the high-frequency voltage-controlled oscillator. For instance, increasing the reverse bias voltage of the variable capacitance diode enlarges the depletion region in the variable capacitance diode, thereby further increasing the distance between the two conductor faces in the variable capacitance diode and thus reducing the capacitance value. Accordingly, the frequency of the LC resonant circuit will be increased. To the contrary, when the reverse bias voltage is reduced, the capacitance value in the variable capacitance diode increases and the frequency of the LC resonant circuit will be decreased. On the other hand, a low-frequency voltage-controlled oscillator uses different control methods based on different frequency ranges, for example, by controlling the current to change the charging speed for the capacitor.
In a CMOS voltage-controlled oscillator of the prior art, such as a CMOS voltage-controlled oscillator disclosed by U.S. Pat. No. 7,196,592, a variable capacitor is made by connecting a capacitor and a variable resistor in series. However, when the required oscillation frequency is increased to some extent (e.g. 60 GHz), due to the reduction in the quality factor (Q) of the variable capacitor, the maximum variable capacitance value tolerated by the same negative impedance circuit is significantly reduced, thus narrowing the adjustable frequency range and making the voltage-controlled oscillator not applicable to broadband systems.
When a higher frequency resolution is required, in a voltage-controlled oscillator using a variable inductor of the prior art, such as a CMOS voltage-controlled oscillator disclosed by U.S. Pat. No. 7,268,634, numerous CMOS switch/capacitor pairs must be employed as its teaching. These CMOS switch/capacitor pairs that need to be fully switched on and off inherently possess a considerable parasitic capacitance, area and a complicated layout, so its oscillation frequency will be limited by the non-ideal effects.